There is increasing demand in fields such as packaging materials for a gas barrier material with low permeability to oxygen and moisture. Gas barrier plastic films known in the art include (1) those in which a molded article is produced from a gas-impermeable macromolecular material such as an ethylene-vinyl alcohol copolymer, a vinylidene chloride copolymer, or a poly-m-xylylene adipamide; (2) those in which one of these gas-impermeable materials is used to laminate or coat another material; (3) those in which an aluminum foil is laminated with a film-forming material; and (4) those in which a metal oxide is vapor deposited.
However, a problem with the ethylene-vinyl alcohol copolymer or poly-m-xylylene adipamide mentioned in (1) is that these materials are highly hygroscopic, and their gas barrier performance decreases as moisture is absorbed, while a problem with a vinylidene chloride copolymer is the pollution caused by the chlorine atoms. The aluminum foil-laminated film of (3) is not transparent, which is a problem in that the contents of a package cannot be viewed from the outside. A problem with the metal-deposited film of (4) is that it is prone to cracking, which can lead to impaired gas barrier performance during packaging.
In an effort to solve these problems, there has been research into silicon oxide thin films obtained by the hydrolysis of a tetraalkoxysilane. A problem with this, however, is that a tetraalkoxysilane hydrolytic condensation reaction results in a large volumetric shrinkage during condensation, which makes the material susceptible to cracks and pinholes. Another area that has been investigated is the suppression of cracks and pinholes by subjecting an alkyltrialkoxysilane to hydrolytic condensation by itself or to co-hydrolytic condensation with a tetraalkoxysilane, but because an alkyltrialkoxysilane has low reactivity, a drawback is that a large amount of monomer remains unreacted. A further drawback is that uniform co-hydrolytic condensation is next to impossible when a tetraalkoxysilane is also used.
In Japanese Laid-Open Patent Application 2-286331, for example, it has been proposed that an alkoxysilane be subjected to hydrolytic condensation to cover a plastic film, but a drawback to this method is that the flexibility of the film is markedly compromised because only the alkoxysilane component coated the film. Also, a composition composed of a monoaminoalkyl group- or diaminoalkyl group-containing alkoxysilane and a compound having an aromatic ring or a hydrogenated ring has been proposed in Japanese Laid-Open Patent Application 7-18221, but the oxygen barrier performance of this composition cannot be considered satisfactory, and needs to be improved further.